How do I focus the camera on a certain area or object?
Tracking

I think the term you wanted to use is "tracking". Focus refers to whether the camera is in-focus or blurred.


Select your Camera
From the Constraints context add a Damped Track Constraint
Set the Target to be your object you want the Camera to follow
Set the axis to -Z (that's the direction the Camera looks out of)




Focus

If you want to set the camera's focus, (as in blur), add an Empty to your scene, then from your Camera settings specify that the Empty is to be used to specify the point of focus. Position the Empty accordingly. Adjust the F-Stop to control the amount of blur.